The long-term evolution of intertidal mudflats in the northern Netherlands during the Holocene; natural and anthropogenic processes

At present, intertidal mudflats form only a minor part of the tidal area of the Wadden Sea. During the Holocene, however, the extent of intertidal mudflats was much larger, as is indicated by abundant mud layers in the coastal sequence of the northern Netherlands. In this paper, a new model for the long-term coastal evolution of the northern Netherlands is presented and the (natural/anthropogenic) origin of the intertidal mudflats is discussed. The Fivel palaeo-tidal basin (NE Groningen) serves as foundation of this model, with time steps in reconstruction of more than 1000 years. Shorter duration events are ignored. The driving force in the coastal evolution during the early Holocene was the rapid sea-level rise. The Pleistocene valley systems of the Northern Netherlands drowned and became tidal basins. In their inner parts intertidal mudflats evolved. The coastline was situated about 10-15 km north of the present-day Wadden Sea islands and the tidal range along the coastline was smaller (microtidal, ca. 1.0 m, 7500 BP, as against mesotidal, 1.8-2.4 m, today). As energy conditions were moderate, the intertidal mudflats were partly covered by salt-marsh vegetation ('rooted mud layers'). Due to increasing tidal range, since about 6200 BP the vegetation cover disappeared from the intertidal mudflats ('non-rooted mud layers'). After 5000 BP, the marshes of the tidal basins started to prograde seaward, mainly as a result of the declining sea-level rise rate. On the other hand, the island coast retreated landward, so that the back barrier land-sea gradient became narrower and steeper. This steeping of the land-sea gradient together with the further increase in the tidal range caused an increase in energy in the tidal basins (local tidal currents and wave action). Because of this natural process, the intertidal sandflats expanded at the expense of the mudflats, and around 2000 BP the sandflats bordered the salt-marsh area, Mudflats were largely lost. Penetration of the sea into the marsh and peat areas was enhanced by human interference, since artificial drainage had led to a significant subsidence and new tidal storage areas were created. After 2000 BP the sea broke into the salt-marsh and peat areas and new tidal systems such as the Dollard (800- 400 BP) were formed. At the back of these 'man-induced' tidal systems intertidal mudflats came into existence. During the late Subatlantic (800 BP-present), these systems silted up to the suyratidal level and were embanked again. The Dollard area, however, was not re-embanked completely. Here, intertidal mudflats still exist at the back of the tidal system. Other artificial mudflats in the Wadden Sea are located in the man-made sedimentation fields along the present sea dike. (C) 2000 Elsevier Science Ltd. All rights reserved.